Asphalt is a flexible adhesive used for pavement construction, which has good adhesiveness, waterproofness, insulation and chemical stability, and is an indispensable material for pavement construction.
The quality of pavement asphalt is usually evaluated by following indexes:
Ductility: mainly used for reflecting low temperature performance of asphalt. Ductility of asphalt refers to the length that when two ends of a standard sample is stretched to get ruptured under specified speed and temperature. Ductility means the rupture length when shear pressure on a shear surface is larger than cohesion of asphalt, which is changed with the change of temperature. Ductility reflects the viscoelastic property of asphalt, and can be measured according to the method specified in ASTMD113 or GB/T4508.
Softening point: reflecting high temperature performance of asphalt. The softening point of asphalt means equiviscous temperature of asphalt under a certain condition; high softening point means that the equiviscous temperature of asphalt is high, and the high temperature stability of mixture is good; and the softening point can be measured based on the method specified in ASTMD36 or GB/T4508.
Penetration number: consistency indicator of asphalt. Penetration number reflects hardness of asphalt under a certain condition, and is indicated by vertically penetrating a standard needle into the depth of asphalt sample under conditions of certain load, time and temperature, the unit is 1/10 nm; the logarithms of penetration number with different temperature are formed into a linear relationship with the temperature, namely IgP=AT+K, where P is the penetration number when the temperature is T; A reflects the sensitivity of penetration number logarithmic temperature, which is called penetration temperature coefficient for short; and K is a constant number.
The rigidity and intensity of asphalt under high temperature are reduced due to the temperature sensitivity of asphalt, and the pavement is softened; while at low temperature, the pavement crack phenomenon such as cracking shrinking and the like is easily to be produced; the pavement must be modified, so as to improve the mechanical performance under high and low temperatures, and this is known by technicians in the field. The modifier used for modifying the asphalt is usually a thermoplastic elastomer polymer, such as random or segmented copolymer of styrene and conjugated diene, polyisoprene, butyl rubber and the like; these modifiers can bestow asphalt excellent mechanical performance and dynamic performance, particularly excellent viscoelasticity; however, the price of the asphalt is high, and a large amount of asphalt is required for use, thus, the cost of modified asphalt concrete is increased obviously.
The waste tire contains various macromolecular polymers such as styrene butadiene rubber, natural rubber and the like and a lot of carbon black, antioxidant, filling material, processing oil and the like, thus, the waste rubber particle can be used as modifier after being activated to add into asphalt. The product obtained by mixing the activated rubber powder and asphalt is called rubber asphalt, and also called asphalt rubber. The activated rubber powder swells in the asphalt, thus, the adhesiveness of asphalt is increased, the oil film thickness is thicker and the overall performance of the rubber asphalt is obviously better than that of the matrix asphalt. The advantage of rubber asphalt is very prominent; however, as the activation of waste rubber powder is complex and a large amount of asphalt is required for use, the rubber asphalt currently has the disadvantages of high cost, complex process and strict requirements.
Generally speaking, the content of rubber powder is higher, the pavement performance of corresponding rubber asphalt mixture is better; however, as the existing rubber asphalt increases the thickness of rubber membrane in the mixture, while the adhesiveness of rubber asphalt is increased, the flowability is reduced, thus, the workability of construction of mixture is reduced, thus, the quality of rubber powder in existing rubber asphalt is strictly controlled at 15-20 percent of the gross mass of the mixture; as the increment of the adhesiveness of rubber asphalt is limited, the asphalt-aggregate ratio (mass ratio of rubber asphalt and ore material) of the mixture in later working section is 8 to 10; the use quantity of asphalt is relative large, thus, the cost of rubber asphalt mixture is high, and it is very uneconomical.
At present, the adopted method for activating waste rubber comprises the following steps of adopting a vulcanizing system by using tert-butyl phenol-formaldehyde resin as vulcanizing agent and using stannous chloride as accelerating agent, and improving the vulcanization speed and viscosity by adopting a halogenated method; secondly solving the rubber adhesive property and vulcanization speed of rubber by using activating agent 420 (Alkyl phenol disulphide) under high temperature and increasing the quantity of stannous chloride, so that the waste rubber powder is vulcanized to restore the crude adhesive attachment property, thereby ensuring the mixed melting property with other materials, and reaching the purpose of modifying other materials. That's to say, the normal method for improving the adhesiveness performance of waste rubber particle is to adopt a desulfuration method to reach the purpose, however, the desulfuration process is complex, the environment is polluted severely, and energy resource is wasted, thus, the method is quite uneconomical.
China patent ZL2005100022110.9 discloses an modified asphalt and preparation method thereof, the modified asphalt can prevent rubber modifier from separating from asphalt, and has the advantages of good softening point, penetration number and ductility, and activation can be directly conducted during preparing activated rubber powder, thus, the waste tire powder is not required to be desulfurated, and the method is simple. However, the modified asphalt cannot effectively solve the contradiction of the adhesiveness and flowability of rubber asphalt; the content of rubber powder in the modified asphalt is still required to be controlled at 15-20% of the gross mass of the rubber asphalt mixture, and the asphalt-aggregate ratio of the mixture in the later working section is still 8-10, a large amount of asphalt is still required.